1. Technical Field
Methods and systems are disclosed herein related to detection of navigation signals based on correlations amongst the navigation signals, and to computation of a navigation solution from the detected navigation signals.
2. Related Art
In satellite-based navigation, a navigation solution is computed from navigation signals received from multiple navigation satellites. A received navigation signal may have a relatively low signal to noise ratio, which may make it difficult to distinguish the signal from noise. Additionally, motion of a receiver or a satellite may impart a Doppler frequency shift to the signal. Atmospheric and/or other environmental conditions may impart propagation delay to the signal. These issues may impact detection accuracy of the navigation signal, which may impact accuracy of a navigation solution.
In a deep-fade global navigation satellite system (GNSS), such as a global positioning system (GPS), signals received from navigation satellites may be detected along two dimensions: delay (or range), and frequency (or Doppler shift). For each received signal, correlation energies of various range-Doppler hypotheses may be calculated and assembled into a range-Doppler correlation grid.
A navigation signal may be detected from a range-Doppler correlation grid by identifying the range-Doppler hypothesis having the highest grid energy, and comparing the highest grid energy to a threshold. The threshold may be set sufficiently high to minimize false alarms. If the maximum grid energy is greater than the threshold, the corresponding range-Doppler hypothesis may be considered valid. Navigation signals received from other satellites may be detected in a similar fashion. Where a range-Doppler hypothesis greater than the threshold is identified from each of multiple range-Doppler correlation grids, a navigation solution may be calculated using a least-squares method.
As described above, each of the multiple signals is individually detected based on a relatively high per signal threshold. This may preclude detection of a signal from one or more satellites, which may reduce the accuracy of a navigation solution or preclude determination of a navigation solution.